Senior British diplomatic and military staff gave Tony Blair explicit warnings three years ago that the US was disastrously mishandling the occupation of Iraq, according to leaked memos.

John Sawers, Mr Blair's envoy in Baghdad in the aftermath of the invasion, sent a series of confidential memos to Downing Street in May and June 2003 cataloguing US failures. With unusual frankness, he described the US postwar administration, led by the retired general Jay Garner, as "an unbelievable mess" and said "Garner and his top team of 60-year-old retired generals" were "well-meaning but out of their depth".

That assessment is reinforced by Major General Albert Whitley, the most senior British officer with the US land forces. Gen Whitley, in another memo later that summer, expressed alarm that the US-British coalition was in danger of losing the peace. "We may have been seduced into something we might be inclined to regret. Is strategic failure a possibility? The answer has to be 'yes'," he concluded.

The memos were obtained by Michael Gordon, author, along with General Bernard Trainor, of Cobra II: the Inside Story of the Invasion and Occupation of Iraq, published to coincide with the third anniversary of the invasion.

The British memos identified a series of US failures that contained the seeds of the present insurgency and anarchy.

The mistakes include:

· A lack of interest by the US commander, General Tommy Franks, in the post-invasion phase.

· The presence in the capital of the US Third Infantry Division, which took a heavyhanded approach to security.

· Squandering the initial sympathy of Iraqis.

· Bechtel, the main US civilian contractor, moving too slowly to reconnect basic services, such as electricity and water.

· Failure to deal with health hazards, such as 40% of Baghdad's sewage pouring into the Tigris and rubbish piling up in the streets.

· Sacking of many of Saddam Hussein's Ba'ath party, even though many of them held relatively junior posts.

Mr Sawers, in a memo titled Iraq: What's Going Wrong, written on May 11, four days after he had arrived in Baghdad, is uncompromising about the US administration in Baghdad. He wrote: "No leadership, no strategy, no coordination, no structure and inaccessible to ordinary Iraqis."

He said the US needed to take action in Baghdad urgently. "The clock is ticking." Both Mr Sawers, who is now political director at the Foreign Office, and Gen Whitley see as one of the biggest errors a decision by Donald Rumsfeld, the US defence secretary, and General Tommy Franks, the overall US commander, to cut troops after the invasion.

Mr Sawers advocated sending a British battalion, the 16th Air Assault Brigade, to Baghdad to help fill the gap. Although the US supported the plan, Downing Street rejected it weeks later.

The British diplomat is particularly scathing about the US Third Infantry Division, which he describes as "a big part of the problem" in Baghdad. He accused its troops of being reluctant to leave their heavily armoured vehicles to carry out policing and cites an incident in which British Paras saw them fire three tank rounds into a building in response to harmless rifle fire.

Mr Sawers, who had been British ambassador to Egypt before being sent to Iraq and is at present on a shortlist to be the next ambassador to Washington, sent the memo to Mr Blair's key advisers, including Jonathan Powell, the No 10 chief of staff, and Alastair Campbell, head of the Downing Street press operation at the time.

Mr Sawers, in later memos, welcomed the replacement of Gen Garner with Paul Bremer, a US diplomat. But in a memo written in June 25, Mr Sawyer concluded that, despite Mr Bremer's arrival, the situation was getting worse.

In that memo, Mr Sawers expressed opposition to further troop reductions. "Bremer's main concern is that we must keep in-country sufficient military capability to ensure a security blanket across the country. He has twice said to President Bush that he is concerned that the drawdown of US/UK troops had gone too far, and we cannot afford further reductions," Mr Sawers said.

Throughout his time in Iraq, however, Mr Sawers remained optimistic Mr Bremer would make a difference.

His views in the memo are echoed in a note by Gen Whitley, who says that while Gen Franks took credit for the fall of Baghdad, he showed little interest in the postwar period. "I am quite sure Franks did not want to take ownership of Phase IV," Gen Whitley wrote.

He added that Phase IV "did not work well" because the concentration was on the invasion. "There was a blind faith that Phase IV would work. There was a failure to anticipate the extent of the backlash or mood of Iraqi society."